1. Technical Field
This invention relates to fiber reinforced plastic parts and, more particularly, to techniques for forming hollow, lightweight parts with complex shapes.
2. Discussion
The present invention is particularly well suited for making vehicle body closures such as doors, lift gates, deck lids, and hoods. Most typically, automotive body closures are now fabricated from separate outer and inner panels made of metal. These panels are joined together with a combination of welding, hem flanging, mechanical fastening and bonding. The joining operation generally requires expensive fixturing to accurately join the inner and outer panels in addition to the need for two tool and die sets to make the panels themselves. It has been recognized that some advantages can be obtained by making the outer panel from fiber reinforced plastic. However, there still generally must be another joining process to assemble the outer panel to the inner panel, trim panel and separate mirror stalk.
It is, of course, necessary to maintain a smooth, aesthetically pleasing appearance for the outer surface of the outer panel. This is generally known as the "Class A" surface in the trade. The use of welding to join together the various parts can damage the Class A surface. Rigid adhesives, can also cause problems with "read through" of the bond line, if such adhesives are used as alternatives to welding. Unless the inner and outer panels are rigidly joined they do not work well together in carrying loads thereby resulting in an unacceptable structure.
Still other problems with the typical method of manufacturing vehicle body closures is the number of panels and parts that must be manufactured. Multiple pieces lead to assembly variation, productivity losses and quality-control problems.
Commonly assigned U.S. patent application Ser. Nos. 768,259, filed Aug. 22, 1985 (now abandoned); 136,187, filed Dec. 21, 1987 (now U.S. Pat. No. 4,863,771); 833,304, filed Feb. 26, 1986 now U.S. Pat. No. 4,740,346; and 190,055, filed May 4, 1988 (now U.S. Pat. No. 4,849,147) disclose various methods of forming hollow fiber reinforced plastic structures. These applications generally disclose, in at least one embodiment, the use of an inflatable bladder that can be pressurized to define the hollow interior of the part when resin is injected into the mold. The bladder can be envisioned as a type of balloon. It is not self-supporting unless it is pressurized. In accordance with the disclosures in these applications, fibrous pieces are first laid in one surface of the open mold and then the bladder is placed on these pieces. Then, the fibrous pieces for the top of the part are laid over the bladder. The mold is closed and then the bladder is pressurized during the resin injection step. While this method has provided satisfactory results, it does have some drawbacks. For example, problems are sometimes encountered because the balloon-like bladder does not completely expand as anticipated when pressurized thereby incompletely defining the interior of portions of the part. The mold loading procedure was somewhat awkward in that most operations are performed at the mold site. In addition, the fibrous pieces on the top of the bladder have a tendency to sag downward thereby causing some unwanted shifting thereof until the mold is closed and the bladder later pressurized.